The invention relates to marine propulsion device tilt and trim mechanisms, and, more particularly, to hydraulic tilt and trim mechanisms which contain a compressible fluid or gas.
Borst U.S. Pat. No. 3,863,592 discloses a combined damping and lift means for a marine propulsion device. This means includes a hydraulic cylinder-piston assembly connected between the transom bracket and swivel bracket of a marine propulsion device, with the cylinder containing a compressible fluid or gas. The function of the hydraulic assembly is to provide an upward force on the swivel bracket, thereby assisting manual upward tilting of the propulsion unit, and also to damp downward tilting movement of the propulsion unit. The hydraulic assembly alone cannot be used to set the bottom trim position of the propulsion unit since it does not limit downward tilting movement of the propulsion unit. Accordingly, the marine propulsion device is provided with a conventional trim adjustment or thrust pin for setting the trim position of the propulsion unit.
Iwashita U.S. Pat. No. 4,493,659 discloses a hydraulic cylinder-piston assembly with the cylinder containing a compressible fluid or gas. The hydraulic assembly also includes a by-pass valve for permitting fluid flow between the cylinder chambers. When the by-pass valve is open, fluid can flow freely between the cylinder chambers, and the propulsion unit can be manually tilted. FIGS. 14-17 of the Iwashita patent illustrate an embodiment which allows the operator to selectively lock the drive unit in any tilt position. However, the hydraulic assembly has a complicated construction, including a free-floating disc 236, and the cylinder chamber 46 that opposes contraction of the hydraulic assembly contains gas. This results in an unstable or "mushy" trim setting.
Attention is also directed to the following U.S. Pat. Nos.:
______________________________________ Wenstadt 4,605,377 08/12/86 Nakase 4,565,528 01/21/86 Hall 4,096,820 06/27/78 Frank 3,008,445 11/14/61 Anderson 3,029,770 04/17/62 Herreman 3,039,724 06/19/62 Hall 3,983,835 10/05/76 Hall, et al. 4,064,824 12/27/77 Nakahama 4,521,202 06/04/85 Nakahama, et al. 4,545,769 10/08/85 Iwashita, et al. 4,551,104 11/05/85 Nakahama, et al. 4,575,342 03/11/86 ______________________________________